Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-03-29
2002-10-01
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S094000, C429S245000, C429S324000
Reexamination Certificate
active
06458485
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to nonaqueous electrolyte secondary cells which comprise a can and a rolled-up electrode unit housed in the can and serving as an electricity generating element, and more particularly to nonaqueous electrolyte secondary cells which are adapted to promptly suppress a current exceeding a predetermined value when the excessive current occurs.
BACKGROUND OF THE INVENTION
In recent years, attention has been directed to lithium secondary cells or batteries which are adapted for a greater capacity and higher energy density for use as power sources for electric motor vehicles or hybrid cars. For example,
FIGS. 5 and 6
show a cylindrical lithium secondary cell which comprises a cylindrical cell can
1
having a cylinder
11
and lids
12
,
12
welded to the respective ends thereof, and a rolled-up electrode unit
2
encased in the cell can
1
. A pair of positive and negative electrode terminal assemblies
9
,
9
are attached to the lids
12
,
12
, respectively. The rolled-up electrode unit
2
is connected to the terminal assemblies
9
,
9
by a plurality of current collector tabs
3
, whereby the electric power generated by the electrode unit
2
can be delivered to an external device from the pair of terminal assemblies
9
,
9
. Each lid
12
is provided with a gas vent plug
13
.
With reference to
FIG. 7
, the rolled-up electrode unit
2
comprises a positive electrode
23
containing a lithium containing composite oxide, a negative electrode
21
containing a carbon material, and a separator
22
impregnated with a nonaqueous electrolyte and interposed between the electrodes, the assembly of these components
21
to
23
being rolled up into a cylinder. A plurality of current collector tabs
3
outwardly extend from each of the positive electrode
23
and the negative electrode
21
of the unit
2
, and the outer ends
31
of the current collector tabs
3
of the same polarity are joined to one electrode terminal assembly
9
. For convenience' sake, only some of these tabs are shown as being joined at their outer ends to the terminal assembly
9
in
FIG. 6
, while the outer ends of the other tabs connected to the assembly
9
are omitted from the illustration.
The electrode terminal assembly
9
comprises a screw member
91
extending through a hole in the lid
12
of the cell can
1
and mounted on the lid
12
. The screw member
91
has a flange
92
at its base end. An insulating packing
93
is fitted in the hole of the lid
12
for electrical insulation and effective sealing. The screw member
91
has a washer
94
fitted therearound from outside the cylinder
11
, and a first nut
95
and a second nut
96
screwed thereon similarly. The first nut
95
is tightened up to clamp the insulating packing
93
between the flange
92
of the screw member
91
and the washer
94
and thereby seal off the hole more effectively. The outer ends
31
of the current collector tabs
3
are secured to the flange
92
of the screw member
91
by laser welding or ultrasonic welding.
For connecting the current collector tab
3
to the negative electrode
21
or positive electrode
23
of the rolled-up electrode unit
2
, a current collector strip forming the electrode and coated with an electrode material over a surface has a known structure comprising a portion of the surface not coated with the electrode material and having the base portion of the current collector tab secured thereto by laser welding or ultrasonic welding (JP-A No. 267528/1994).
When the nonaqueous electrolyte secondary cell described develops a short-circuit in its interior, a great current is likely to flow. To avoid such an incidence, an electrode structure has been proposed. For example, a positive electrode
8
comprises, as shown in
FIG. 8
, a current collector
81
which is provided with an electrode material
83
over each of its opposite surfaces, with a PTC (positive temperature coefficient) element layer
82
formed therebetween (JP-A No.220755/1995). The PTC element providing the layer
82
has a positive temperature coefficient of resistance, such that when a current in excess of a predetermined value flows therethrough, the electric resistance value of the element rapidly increases to exhibit a current suppressing effect. When the secondary cell having the proposed electrode structure develops an inside short-circuit, a current exceeding the predetermined value will not flow continuously.
However, in the case of the conventional nonaqueous electrolyte secondary cell having the PTC element layer
82
shown in
FIG. 8
, the presence of the PTC element layer
82
between the current collector
81
and the electrode material
83
makes the quantity of the electrode material
83
correspondingly smaller than otherwise per unit volume of the cell can, consequently entailing the problem of greatly reducing the discharge capacity per unit volume of the cell can, i.e. energy density.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nonaqueous electrolyte secondary cell which is adapted to prevent continuous occurrence of a current exceeding a predetermined value and realize a high energy density.
The present invention provides a nonaqueous electrolyte secondary cell which has an electrode unit
2
housed in a cell can
1
and comprising a positive electrode and a negative electrode each including a current collector. The cell is characterized in that the current collector of at least one of the positive electrode and the negative electrode comprises a plurality of current collector pieces
42
arranged along one direction and a PTC element
5
interconnecting each pair of adjacent current collector pieces
42
.
With the secondary cell of the invention stated above, the PTC element
5
is interposed between each pair of adjacent current collector pieces
42
and can therefore be given a minimum length necessary for interconnecting the collector pieces
42
. This greatly reduces the volume occupied by such PTC elements
5
in the interior of the cell can
1
unlike the conventional construction wherein a PTC element layer is interposed between the current collector and the electrode material, consequently permitting use of a larger amount of electrode material than conventionally to result in an energy density as high as is the case with cells having no PTC element.
Stated more specifically, end portions of the pair of current collector pieces
42
,
42
to be interconnected are lapped over each other, with the PTC element
5
held therebetween, and joined to respective opposite surfaces of the PTC element
5
. With this specific structure, the joint between the PTC element
5
and each current collector piece
42
can be given a sufficiently large area, whereby the adjacent current collector pieces
42
can be connected to each other firmly.
The number A of current collector pieces
42
, the overall length B of the electrode along the winding direction of the electrode unit
2
and the length C of the PTC element
5
have the relationship represented by the following expression:
A×C/B<
0.1
When this relationship is established, a much greater discharge capacity than in the prior art is available as will be substantiated by experimental results to be described later.
The present invention further provides another nonaqueous electrolyte secondary cell including an electrode unit
2
housed in a cell can
1
and comprising a positive electrode and a negative electrode each provided by coating a surface of a striplike current collector
61
with an electrode material
62
to form coated portions and an uncoated portion not coated with the electrode material
62
, a current collector tab
3
having a base end portion connected to the uncoated portion and an outer end portion connected to an electrode terminal assembly
9
. At least one of the positive electrode and the negative electrode is provided with a PTC element
7
held between opposed faces of the uncoated portion of the current collector
61
thereof and the base
Chikano Yoshito
Funahashi Atsuhiro
Maeda Takeshi
Nishio Koji
Nohma Toshiyuki
Kubovcik & Kubovcik
Maples John S.
Sanyo Electric Co,. Ltd.
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